Carrier frequency shift selecting system



Nov. 1, 1949 c. BUFF 2,486,920

CARRIER FREQUENCY SHIFT SELECTING SYSTEM Filed March 15, 1946 CHR/Ls'ToPI/ER flu INVENIOR.

BY /WM ATTORNEY Patented Nov. 1, 1949 CY SHIFT SELECTING YSTEM CARRIER FREQUg-N Christopher Bud, Hicksville, N. Y., assignor, by

mesne assignments, to J efierson Standard Broadcasting Company, a corporation of North Carolina Application March 13, 1946, Serial No. 654,055

6 Claims.

This invention relates to frequency shift keying systems and more particularly to frequency shift keying systems for producing a substantially constant frequency shift voltage output notwithstanding wide amplitude variations in the input keying voltage.

It is usually desirable that a frequency shift keying system produce a substantially constant frequency shift voltage output, thereby insuring proper operation of the apparatus which is keyed by it, such as for example a radio transmitter. In order that the keying system produce such an output, it is necessary to provide means to make the frequency shift voltage output independent of variations in the amplitude or keying level of the input voltage. Since some uses require the association with the keying system of circuits such as frequency multipliers which require resetting of the frequency shift for each such circuit, it is desirable that means be provided in the keying system to eliminate the necessity of a change in the frequency shift setting for each such association.

Accordingly, it is a principal object of this invention to provide a frequency shift keying system for producing substantially constant frequency shift keying voltages despite wide amplitude or level variations in the input keying voltage.

It is another object of this invention to provide a a frequency shift keying system having substantially constant frequency shift excursion and which may be associated with circuits such as frequency multipliers without necessitating a resetting of the frequency shift of the system as a result of such association.

It is a further object of this invention to provide a frequency shift system which produces a series of substantially constant frequency shift voltages, each of which has a predetermined value relative to the fundamental frequency shift voltage.

A feature of this invention relates to the frequency shift voltage output wherein certain positive and negative input voltages cause a fundamental shift voltage of substantially constant value to appear or disappear.

Another feature relates to the circuit arrangement wherein the output voltage is zero when the input voltage is a certain voltage value, and any input voltage having a value negative with respect to that of the minimum threshold voltage, causes no change in the output voltage.

Another feature relates to the circuit arrangeminimum threshold l 2 ment wherein the shift voltage and frequency shift excursions remain constant for all values of input voltage greater than the value of the minimum threshold voltage.

A further feature relates to the output frequency shift divider wherein the fundamental frequency shift excursion can be divided into any desired number of sub-divisions.

A still further feature relates to the novel organization, arrangement and interconnection of parts which cooperate to produce an improved frequency shift keying system having a substantially constant frequency shift output excursion over a wide range of variations in the input keying voltage.

Other objects, features and advantages of this invention will be apparent from the following description taken in connection with theaccompanying drawing which is a circuit diagram of an embodiment showing the inventive features.

Referring more particularly to the drawing, input keying voltages from a suitable source I of keying voltage is applied across input terminals 2-2 and develops corresponding voltages across input load resistor 3. An output frequency shift voltage of substantially constant value is applied over conductor 4 to the control electrode of a suitable reactance tube 5. Tube 5 may be such as disclosed in U. S. Patent No. 2,299,937 and is associated with the frequency-determining element or elements 6 of a carrier oscillator 1, so as to produce a linear relation between input control voltage and reactance variation. The tube 5 should be such that it does not draw grid current over the entire range of control voltage applied to its control grid. Conductor 4 is connected to multi-contact rotary switch 8 which is associated with voltage divider 9 comprising series connected resistances Ill-l1.

Keying voltages appearing across resistor 3 are applied by conductor It to the cathodes i9 of dual half-wave rectifier tube 20. Anodes 2| of tube are connected by conductor 22 to the control grid 23 of triode tube 24. Cathode 25 of tube 24 is connected to ground through series resistors 26 and 27. Conductor 28 connects potentiometer arm 29 associated with adjustable resistor 21 to resistor ll] of divider 9. Anode 30 of tube 24 is connected to ground through series resistors 3| and 32. Potentiometer arm 33 associated with resistor 32 is connected to the control grid 23 of tube 24 through resistor 34. Conductor 35 connects anode 30 to a suitable source of D. C. potential such as power supply 3 36 having a gaseous voltage stabilizer tube 31 associated therewith.

The output impedance of source I should be equal to or less than the resultant impedance presented by resistor 3, tube 20, and resistor 34, so as to prevent a :decrease in the voltage of source I when it is connected to terminals 2-2. When no signal input is present across resistor 3, contact 33 and resistor 32 control the bias on grid 23. When the contact 33 is at one extreme of resistor 32 the plate current of tube '24 is at a maximum e. g., milliamperes. When it is at the opposite end of resistor .32, the plate current may for example be 2 milliamperes. This control of contact 33 is for the purpose of adjusting the maximum voltage at cathode required for the desired frequency shift when contact 29 is at point 38 of resistor 21.

The various resistors connected to control 'grid 23 and cathode 25 are chosen so that control *grid 23 is always "negative with :respect to its :cath'ode 25. If a signal from tube 2i having a greater negative value than the predetermined minimum threshold value is impressed on control grid 23, that grid becomes biased negatively with respect to ground and causes the potential of cathode 25 to be reduced to zero, resulting in tube 24 being at plate-to-cathode current cut- 'oiT.

Because anodes 21 of tube 20 are maintained :at a positive D. 0. potential, tube '29 being normally .plate-current-conductive, the resultant positive potential developed at cathodes 19 with respect to ground must be overcome by the signals from source I. Resistor 34 serves to prevent the drawing of excessive current by grid 23 or by tube 28, when normally conductive, thus limiting the current flowing through rectifier 2t into source I to a low value, such as for example one to "two milliamperes.

For the purposes of the description, suppose that the minimum threshold value of the voltages from source I is +50 or '50 volts. This represents the value of the input signal of 50 volts required to cut off plate current conduction through tube 24 and -{-5'O volts required to cut on plate current conduction through tube 29. Since tube 24 "is at anode-to-cathode :current cut-off when an input signal :of -50 volts is present at cathodest9, :a received input signal of greater negative value than -50 volts can have no effect on tube 24. If the input signal is +50 volts or a greater positive value, tube .20 becomes non-conductive, and grid :23 is then biased in accordance with the .positionof contact 33 to-al-low .a predetermined current flow through tube 24 and thus developing a predetermined voltage across resistor 2:1. From the foregoing, it is clear that for all negative input voltages of 50 volts or greater, there is no voltage developed across resistor 2?, While for all input positive Voltages of 50 volts or greater a predetermined voltage is developed across that resistor.

While description has been made in connection with .a minimum threshold value of +50 01' -5O volts, it is understood that the values of the various circuit elements may be so chosen so that the system similarly operates with a minimum input signal threshold level of as low as +10 or -10 volts.

The fundamental frequency shift voltage appearing across adjustable resistor 27 is applied to divider 9. If it is desired that this fundamental voltage be utilized, switch arm 39 is moved to contact 10. If it is desired that some other voltage be utilized, switch arm 39 is moved to the appropriate one of contacts ll-41'. By proper choice of resistances Ill-l1, any desired fre quency shift ratio may be obtained since that ratio is in exact proportion to the resistance ratio involved. One 'of the important advantages of this arrangement is that the .tfrequency shift system does not have to be reset each time a different number of stages of frequency multiplication are added in the carrier frequency multiplier 48. It is merely necessary to operate switch arm 39 to the particular sub-divider contact 4|4'l which in cooperation with the number of multiplication stages in use, produces the required frequencyrshift in the final carrier output.

While there has been here described a preferred embodiment of this invention, it is understood that various changes and modifications may be made therein without departing from the scope of the invention.

What is claimed is:

1. In a carrier frequency shift system, a source of input signal voltages, a carrier generator whose frequency is to be shifted a predetermined amount to represent said signals; means to derive a carrier frequency shift voltage from said input voltages, the last-mentioned means comprising a grid-c0ntrolled tube, a diode, a connection from a control grid of said tube to said diode, a source of positive D. C. supply, connections between said D. C. supply and the diode and the grid of said tube to apply to said grid a voltage to render said tube normally plate current conductive except when the diode passes current in response to an input signal voltage in excess :of a predetermined minimum, a cathode load resistor for said tube, selective switch means tosel'ect in'snccessive steps a'proportionate part. of the. cathode load voltage in accordance with the frequency shift desired in said carrier, a frequency control device for said carrier generator, and means including said switch means to apply said proportionate part of the cathode load voltage to said frequency control device to =correspondingly shift the frequency of said generator, and a frequency multiplier supplied by said generator arranged to multiply the generated frequency in successive stages the number of said stages being correlated with the number of selective steps of said switch to maintain the per cent of frequency shift in the final carrier constant for any given input signal voltage from said source and for any desired number of said stages.

2. In a carrier frequency shift system, a source of input signal voltages, a diode on which said input voltages are impressed, means normally biasing said diode so that it is conductive only in response to input signal voltages in excess of a predetermined threshold level, a grid-controlled tube having its control grid connected to said diode and normally biased to be conductive between anode and cathode only When said input signals are below said threshold level, a cathode load resistor for said tube, a carrier generator whose frequency is tobe shifted a predetermined amount under control of said input signals, a reactance tube connected to said generator for controlling its frequency, and selective switch means connecting said reactance tube to said cathode load resistor to select a frequency shift voltage in successive stages proportionate to the extent of frequency shift desired in the carrier, and a frequency multiplier supplied 'by said generator arranged to multiply the generated frequency in successive stages the number of said stages being correlated with the number of selective steps of said switch to maintain the per cent of frequency shift in the final carrier constant for any given input signal voltage from said source and for any desired number of said stages.

3. In a carrier frequency shift system, a source of input signal voltages, a carrier generator whose frequency is to be shifted a predetermined amount to represent said signals, a diode on which the input signals are impressed, a grid-controlled tube, a source of D. C. supply, means to bias the control grid of said tube jointly under control of the conductivity of said diode and under control of an adjustable connection to said source of supply, said control grid being biased to render said tube anode-to-cathode conductive only when said input signals are below said predetermined threshold level requisite to render said diode conductive, a cathode load resistor for said tube, a voltage divider resistor having a plurality of connection points, a potentiometer connection between said cathode load resistor and said voltage divider, a reactance tube for controlling the frequency of said generator, and a multi-point selector switch for connecting successive sections of said voltage divider to said reactance tube to select a sub-divided carrier shift voltage to effect the desired frequency shift in said carrier, and a frequency multiplier supplied by said generator arranged to multiply the generated frequency in successive stages the number of said stages being correlated with the number of said successive sections of said voltage divider to maintain the per cent of frequency shift in the final carrier constant for any given input signal voltage from said source and for any desired number of said stages.

4. In a carrier frequency shift system, a source of input signal voltages, a carrier generator whose frequency is to be shifted a predetermined amount to represent said signal voltages, a source of D. C. power supply, a voltage divider connected to said power supply, a diode having its anode adjustably connected to said voltage divider, a grid-controlled tube having its control grid also adjustably connected to said voltage divider, means including said voltage divider and said diode for normally biasing the control grid of said tube to render said tube conductive between anode and cathode only when the input signals applied to said diode are in excess of a predetermined positive threshold level, a cathode load resistor for said tube, a frequency shift voltage sub-divider network adjustably connected to said cathode load resistor, a reactance tube connected to said generator to control its frequency, a multi-point selector switch connected to a plurality of separate points in said voltage sub-divider, said switch having a contact arm for selectively connecting said reactance tube to any desired point in said frequency shift voltage divider, and a frequency multiplier supplied by said generator arranged to multiply the generated frequency in successive stages the number of said stages being correlated with the number of said separate points in said voltage divider to maintain the per cent of frequency shift in the final carrier constant for any given input signal voltage from said source and for any desired number of said stages.

5. In a carrier frequency shift system, a source of input signal voltages, a carrier generator whose frequency is to be shifted a predetermined amount to represent said signals, means to multiply the frequency of said carrier in successive stages; a diode and a grid-controlled tube upon which the input voltages are impressed, said grid-controlled tube having a cathode load resistor, means to bias the control grid of said tube so that it passes anode-to-cathode current through said load resistor only when the input signals are in excess of a predetermined level, means to select from said cathode load voltage a fundamental voltage representing a desired maximum carrier frequency shift, a reactance tube connected to said generator to control its frequency, and selective switch means for selecting from said fundamental shift voltage a sub-divided voltage in accordance with the number of stages of multiplication to which said carrier frequency is subjected, and for applying said sub-divided voltage to said reactance tube.

6. A frequency shift selecting system for producing substantially constant frequency shift keying output voltages notwithstanding wide variations in the input keying voltage to said system including, a source of input keying voltage and a diode energized by said keying voltage, a grid-controlled electron tube having its control grid connected to the anode of said diode, a source of substantially constant D. C. voltage for applying potentials to the plate and control grid I of said grid-controlled tube and plate of said diode so that negative keying voltages greater than a predetermined value cause said grid-controlled tube to be non-conductive, and positive keying voltages greater than a predetermined value cause a substantially constant output frequency shift voltage to appear across the load in the cathode of said grid-controlled tube, a carrier frequency generator whose frequency is to be shifted in accordance with said keying voltages, a reactance tube connected to said generator for controlling its frequency, a voltage divider resistance connected in the cathode load circuit of said grid-controlled tube, and a multi-contact switch for connecting any desired point of said voltage divider resistance to said reactance tube to correspondingly control the carrier frequency, and a frequency multiplier for said frequency generator said frequency multiplier arranged to multiply in successive steps correlated with the number of points of said voltage divider resistance.

CHRISTOPHER BUFF.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,086,540 Dimond July 13, 1937 2,237,631 Peterson Apr. 8, 1941 2,389,879 Tunick Nov. 2'7, 1945 2,401,629 Finch June 4, 1946 

